One technology that is concerned with electric vehicles is a technology for controlling a traction motor (electric motor) {see, for example, U.S. Pat. No. 5,367,455 (hereinafter referred to as “U.S. Pat. No. 5,367,455 A”) and Japanese Laid-Open Patent Publication No. 06-121405 (hereinafter referred to as “JP 06-121405 A”)}.
According to U.S. Pat. No. 5,367,455 A, the maximum speed of an electric automobile is limited on the basis of the running state, operating state, and remaining battery energy level of the vehicle (see column 6, line 17 through column 8, line 14, and FIGS. 2 through 10). Specifically, it is established whether the maximum speed or the acceleration or both are to be limited on the basis of the running state and the operating state or not (column 6, line 64 through column 7, line 7, FIG. 10). In addition, if the maximum speed is to be limited, then a maximum speed Vmax is set depending on the remaining battery energy level DOD {column 6, lines 29 through 35, FIG. 7(a)}. If the remaining battery energy level DOD drops to a preset value DOD′ (S31 in FIG. 3: YES) and if the driver enters a signal indicating that the running performance is to be limited (S34: YES), then the maximum speed is limited depending on settings (after S39: YES) (column 7, line 44 through column 8, line 14).
According to JP 06-121405 A, there is disclosed an electric automobile that is capable of selecting a power mode with emphasis on the driving performance and an economy mode with emphasis on the highly efficient driving (see Abstract, [0006]). Specifically, in the power mode according to JP 06-121405 A, a field current is set depending on the rotational speed of an AC motor. In the economy mode, a field current is set depending on a torque command (see Abstract). The power mode and the economy mode are switched over using a mode switcher 13 ([0007]) or an automatic mode switcher 22 ([0012]).
If the mode switcher 13 is used, then a low-pass filter 20 is used in order to prevent a field current command from changing abruptly ([0010]). For switching from one driving mode to another, the automatic mode switcher 22 weights the economy mode and the power mode depending on an accelerator pedal depressed amount xa and a brake pedal depressed amount xb ([0012]). In addition, a membership function can be used for smoothly switching from one driving mode to another ([0012]).
It is customary to provide three or more driving modes {see, for example, Japanese Laid-Open Patent Publication No. 2007-302055 (hereinafter referred to as “JP 2007-302055 A”)}.